With reference to FIG. 1, a conventional fan failure detector 100 is mainly composed of a motor 110, a drive unit 120 and a rotation speed detecting unit 130. The drive unit 120 is electrically connected to the motor 110 and the motor 110 is capable of being driven by the drive unit 120, the rotation speed detecting unit 130 is electrically connected to the drive unit 120, the rotation speed detecting unit 130 means for detecting the rotation speed of the motor 110, and the drive unit 120 outputs an inspection signal to a monitoring apparatus 200 to determine operating status of the motor 110. When the motor 110 operates normally, the inspection signal can be a continuous pulse signal, oppositely, when the motor 110 locks, the inspection signal can be a surge or a high voltage or a low voltage signal depends on internal predetermined value of the drive unit 120 to make the monitoring apparatus 200 to execute shut down or restart function. Once the monitoring apparatus 200 continuously receives at least two surge signals, the continuous surge signal is likely mistaken to be the continuous pulse signal so that the monitoring apparatus 200 might make a wrong judgment that the motor 110 still operates normally. Or, when the monitoring apparatus 200 receives the high voltage or the low voltage signal, exchange of magnetic poles of the motor 110 causes exchange between high voltage level and low voltage level thereby enabling the monitoring apparatus 200 make a wrong judgment that the motor 110 still operates normally.